Production of metal strip from metal powders



D. K. WORN June 13, 1961 PRODUCTION OF METAL STRIP FROM METAL POWDERS Filed July 30, 1957 United States Patent 2,987,778 PRODUCTION OF METAL STRIP FROM METAL POWDERS David Kenneth Worn, Solihull, England, assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed July 30, 1957, Ser. No. 675,110 Claims priority, application Great Britain Aug. 3, 1956 3 Claims. (Cl. 1859.3)

The present invention relates to the process of rolling metallic strip and, more particularly, to improvements in the process of rolling metallic strip directly from powdered material.

It is known that metal strip can be made from metal powder by passing the powder through rolls arranged side by side with their axes horizontal. The metal powder is fed downwards to the gap between the rolls and passes vertically through it. Thereafter, the compacted powder is sintered. There is, of course, a limit to the width of the gap in that if it is too wide, powder will pass through it without being compacted at all. Provided however that the gap is narrow enough for all the powder to be compacted, the output should apparently increase with increases in the gap and the linear speed of the rolls.

Considering a conventional gravity powder feed to the gap between a pair of horizontally disposed rolls, the maximum thickness, and therefore weight per foot of strip produced, is limited by the onset of laminar cracking or splitting which, with any given powder and with any given mill and with any given roll gap, occurs when a given thickness is reached. Laminar cracking sets a natural upper limit to the thickness of strip which can be produced at any rolling speed with any given mill, with any given roll gap and with any given powder. This cracking is associated with the relief of internal stress in the strip as it emerges from the roll gap. The maximum thickness is readily attained with low rolling speeds, merely by increasing the head of powder above the roll gap; but as the rolling speed is increased above a certain value, an increase in powder head affects only very minor in creases in strip thickness and, in general, the maximum thickness of strip (limited by the onset of laminar cracking) cannot be attained in a simple manner at these higher rolling speeds. Improvements can be obtained at high rolling speeds by feeding the powder under mechanical pressure, namely, by use of very high powder heads or screw feed mechanisms but these methods lead to complications in the design of plant for commercial production.

For reasons of economy, it is desired that a mill produce strip at or near the maximum thickness at the greatest possible speed. Above a certain limit, however, increase in rolling speed brings about a reduction in the thickness and weight per foot of strip which can be attained and thus a rate of production considerably less than would normally be expected with an increase in rolling speed. In addition, the mechanical properties of the green strip (i.e., strip which has been compacted but has received no heat treatment) fall markedly.

Although attempts were made to overcome the foregoing diificulties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by feeding metal powder to therolls under special conditions the rate of production of metallic strip therefrom may be substantially increased.

It is an object of the present invention to provide an improved process for the production of metallic strip from powdered materials by rolling.

Another object of the invention is to provide an imice:

2 proved process for rolling nickel strip from nickel powder.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a diagram showing powder compacting rolls in cooperative relationship with an enclosed powder feed hopper in accordance with the present invention; FIGURE 2 depicts in schematic outline a variation in the apparatus depicted in FIGURE 1, wherein provision is made for the continuous feeding of powder to compacting rolls; and

FIGURE 3 shows the relationship between strip thickness, powder head and roll speed using an embodiment of the present invention as compared to prior practice.

Generally speaking, the present invention contemplates a process for producing metal strip from metal powder which comprises continuously feeding metal powder to cooperating compacting rolls, continuously rolling said powder between said rolls while simultaneously controlling the atmosphere associated with the powder to minimize levitating forces acting on the powder feeding to said rolls and thereafter sintering or otherwise processing the compacted strip produced by said rolling.

It has been found that the powder above the roll gap is subjected to upward levitating forces from entrained air which is expelled as the powder is subjected to the nip of the rolls. These levitating forces adversely aflect the quantity of powder fed into the rolls by diluting the powder adjacent to the nip with the expelled air. This effect becomes more pronounced with increase in rolling speed and is particularly pronounced when using fine powders which readily become air-borne.

In carrying the invention into practice, it is preferred to reduce or substantially minimize the efiect of the aforementioned levitating forces and thus increase the rate of production from any given mill by replacing the air above the roll gap in said mill in association with the metal powder by a gas of low viscosity, particularly a gas having a viscosity substantially lower than the viscosity of air. At room temperature the viscosity of air is about 180 10- poise. The air above the roll gap in associa tion with the powder is advantageously replaced by a gas which has a room temperature viscosity below about x10 poise; and which has no deleterious effect on the powder rolled or the strip produced. Hydrogen, with a room temperature viscosity of about 88 10- poise, is particularly suitable. The hydrogen or other low viscosity gas which has substantially no deleterious effect on the powder being rolled appears to escape more readily from the nip of the rolls and to flow round the powder particles more easily without lifting them to the same extent as air.

Particularly advantageous results are obtained when the air above the roll gap in association with the powder is replaced by hydrogen in an apparatus such as depicted in the drawing in either FIGURE 1 or FIGURE 2. Referring now to the drawing, in the apparatus shown in FIGURE 1, powder is fed to the nip between horizontally paired compacting rolls 10 and 11 from hopper 12 to form a compacted strip 13. Hopper 12 is closed by cover 14 and initially hydrogen is introduced through duct 15. The hydrogen drives out the air through duct 16 which is controlled by valve 17 which is initially open. After air has been purged from hopper 12, valve 17 is closed. Rolls 10 and 11 are actuated to rotate in the indicated directions, thus forming compacted strip 13. 7 Duringthe rolling operation, the supply of hydrogen is maintained through duct 15. Through the use of this apparatus, it has been found that uniform high quality of strip may be maintained at considerably higher roll speeds than could be used were hydrogen not used in the hopper. Although hydrogen may be conveniently maintained at atmospheric pressure during the rolling operation, it has been found that a small increase in the pressure of hydrogen, for example, an increase in pressure equivalent to 1 or 2 centimeters of water pressure above atmospheric, produces a surprising further increase in the thickness and rate of production of the strip produced. Thus a small positive pressure of hydrogen may be used at very high rolling speed to obtain the maximum strip thickness without recourse to high powder heads.

In the apparatus shown in FIGURE 2, which is a variation of the apparatus shown in FIGURE 1, powder is fed from the base of hopper 18 by a screw feed mechanism 19 to casing 20 which is situated directly above rolls 10 and 11. Hydrogen is fed to casing 20 through entrance port 21 and escapes from casing 20 through outlet 22. To insure that the powder head above the roll gap between rolls 10 and 11 remains substantially constant, the speed of the screw feeding mechanism 19 may be controlled by an instrument which includes probes 23 which are immersed in the powder in casing 20 at crictically high and low levels above the roll gap and which are responsive to the presence or absence of powder above them. When rolls 10 and 11 are actuated to rotate in the indicated directions, compacted strip 13 is formed. Of course, a supply of hydrogen is maintained flowing through casing 20 during the entire rolling operation. The supply of hydrogen may have a pressure which is slightly in excess of atmospheric pressure.

The particular advantages of the present invention are illustrated by the graph in FIGURE 3. In FIGURE 3 segmented lines A, B, C and D represent the relationships between strip thickness and powder head at rolling speeds of 2.5, 4.25, 6 and 8 feet per minute in air, respectively. Solid lines X and Y represent the relationship between strip thickness and powder head at rolling speeds of 6 and 8 feet per minute in hydrogen, respectively. The nominal roll gap setting in each aforementioned instance was 0010 inch.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative example is given:

Example I Powder feed apparatus corresponding to that depicted in FIGURE 1 is positioned above counter-rotating compacting rolls which are 7 /8 inches in diameter. Carbonyl nickel powder known as B carbonyl nickel powder is introduced into the powder feed apparatus. Hydrogen is introduced to the hopper at a pressure of l centimeter of water. A rolling speed of eight feet per minute and a powder head 12 cm. produces a strip 0.030 inch thick.

The strip produced directly from rolling the powder, conventionally known as green strip, has a strength which is sufiicient to permit it to be processed further by sintering, annealing and further rolling operations.

In contrast, when using the same apparatus and powder such as disclosed in the aforementioned example, but without the replacement of air with hydrogen, a rolling speed of eight feet per minute produces a strip which is 0.019 inch thick even with a powder head of 16 cm. To produce the same quality strip as produced in Example I, a maximum rolling speed of about 2.5 feet per minute can be used. Thus, by the use of the present invention, a strip thickness, weight per unit length of strip and strength of strip normally obtained when using a rolling speed of 2.5 feet per minute may be maintained at a rolling speed of eight feet per minute. This results in about a 200% increase in productive capacity of a given rolling mill.

It is to be observed that the present invention is particularly applicable to very light powder, for example, those produced by the thermal decomposition of nickel carbonyl and having a fibrous structure. This powder may have a bulk density of about 1 gm. per cubic centimeter. Strips produced from this type of powder are in keeping with others subject to the defect known as laminar cracking. Of course, while the invention is particularly applicable to very light powders, it may be used to advantage when any powders are being compacted by rolling methods. By the use of the present invention the forces which upset the uniform how of powder to the roll gap are minimized, resulting in more efficient operation of the rolling mill. Although the invention is also particularly applicable to processes in which the rolls are arranged side by side, such as depicted in the drawing, forces similar to those described as levitating forces, influence the feed of powder to the gap between rolls disposed one above the other with horizontal axes lying in a single vertical or inclined plane. The present invention contemplates the reduction of these levitating-like forces by the use of a low viscosity gas with or without the use of a screw feed mechanism.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for producing green .compacted metal strip substantially free from laminar cracks from metal powder having a low bulk density by high speed rolling which comprises displacing air entrained with said metal powder by a gas inert to said metal powder having a viscosity less than about 150x 10- poise and rolling said low bulk density powder to green strip free of laminar cracks while maintaining said powder in contact with said gas having a viscosity less than about 150X10- poise.

2. A process for producing green compacted metal strip substantially free from laminar cracks from metal powder having a low bulk density by high speed rolling which comprises displacing air entrained with said metal powder by hydrogen gas having a viscosity less than about 150x10 poise and rolling said low bulk density powder to green strip free of laminar cracks while maintaining said powder in contact with said hydrogen gas having a viscosity less than about 150x10 poise.

3. A process for producing green compacted metal strip substantially free from laminar cracks from metal powder having a low bulk density by high speed rolling which comprises displacing air entrained with said metal powder by hydrogen gas having a pressure slightly above atmospheric pressure and having a viscosity less than about 150x 10 poise and rolling said low bulk density 2,159,231 Schlecht May 23, 1939 2,287,663 Brassert June 23, 1942 2,378,539 Dawihl June 19, 1945 2,457,861 Brassert Jan. 4, 1949 2,882,554 Heck Apr. 21, 1959 

1. A PROCESS FOR PRODUCING GREEN COMPACTED METAL STRIP SUBSTANTIALLY FREE FROM LAMINAR CRACKS FROM METAL POWDER HAVING A LOW BULK DENSITY BY HIGH SPEED ROLLING WHICH COMPRISES DISPLACING AIR ENTRAINED WITH SAID METAL POWDER BY A GAS INERT TO SAID METAL POWDER HAVING A VISCOSITY LESS THAN ABOUT 150X10-6 POISE AND ROLLING SAID 